The transport functions of the various tubular segments of the nephron have been well characterized in recent years. Urine acidification by the collecting tubule has been recognized to occur by mechanisms distinct from those in the proximal tubule and to differ in the cortical and medullary segments. On the other hand, the metabolic requirements for active transport, especially in the distal nephron, have not been as well studied. The aim of the studies proposed here is to define the metabolic processes which support acidification (or bicarbonate reabsorption) in the collecting tubule. In this regard, the role of oxidative metabolism (versus anaeorbic metabolism) and of various potential metabolic substrates (glucose, lactate, fatty acids, etc.) will be examined in vitro. The subsequent goal is to develop a method to simultaneously quantitate the rate of substrate decarboxylation and the rate of active transport in single in vitro tubular segments. Active H+ transport in the collecting tubule is to be examined in the studies here. Also, the proposed studies aim to define the response of collecting tubule acidification to changes in pC02. All of these studies utilize the technique of in vitro microperfusion in isolated rabbit tubules. Variations in bathing solution pC02, p02 organic metabolic substrates will be used to define the determinants and requirements of acidification by cortical and medullary collecting tubules. Acidification and bicarbonate reabsorption will be measured using pH electrodies and microcalorimetry. Substrate decarboxylation will be assessed by 14C02 (Less Than Greater Than H14C03-) generation. These studies are expected to yield new information on collecting tubule physiology and metabolism and will hopefully provide a new method to examine the relationship of metabolism to active hydrogen transport.